1. Field of the Invention
The present invention relates to a thermoelectric conversion device using a thermoelectric conversion material capable of converting between thermal energy and electric energy, and a cooling method and a power generation method using the same.
2. Description of the Related Art
Thermoelectric conversion is a technology using Seebeck effect in which electromotive force is generated when a temperature gradient is provided to a substance, and Peltier effect in which the temperature gradient is generated when an electric current is applied through the substance.
More specifically, the thermoelectric conversion is a technology using the Seebeck effect in which thermoelectric generation occurs when a temperature difference is provided and the Peltier effect in which thermoelectric cooling occurs when an electric current is applied, in a configuration where two kinds of substances each of which is different in carrier polarity, a p-type semiconductor and an n-type semiconductor, for example, are thermally connected in parallel and electrically connected in series.
Under present circumstances, the technology using the thermoelectric conversion is lower in efficiency compared to technologies using other power generation and cooling methods. Therefore, it is used only in a few special applications such as a power sources for remote areas, a power sources for space, a local cooling such as to be used for an electronic device, and a wine cooler.
The performance of a thermoelectric conversion material used for a thermoelectric conversion device is evaluated by a figure of merit Z, or a figure of merit ZT that is made dimensionless by multiplying Z by absolute temperature T.
ZT is a quantity expressed by ZT=S2T/ρκ where S is a Seebeck coefficient of the substance, ρ is an electric resistivity thereof, and κ is a thermal conductivity thereof. A material with a larger ZT value is excellent as the thermal conversion material.
To date, a thermal conversion material mainly used as a practical application, although used in a special application, is a semiconductor of Bi2Te3.
Bi2Te3, however, has various problems such as instability at a high temperature, toxicity, scarcity of the element.
Ever since it was discovered that a layered oxide of NaxCoO2 was a substance that exhibited a good thermoelectric conversion performance (JP 9(1997)-321346 A (reference 1), and WO03/085748 (reference 2)), material searching has been practiced with efforts in order to discover a layered oxide having a higher thermoelectric conversion performance.
The layered oxide has advantages in that it is stable even in the air at high temperature, etc.
Furthermore, as the main characteristics, the layered oxide offers a strong dimensional anisotropy, and most of the layered oxides have a layered crystalline structure (hereinafter, may be referred to as a layered structure) formed of electric insulating layers and second dimensional electric conductive layers that provide electric conduction.
FIG. 1 shows the crystalline structure of NaxCoO2 in which electric conductive layers composed of CoO2 and electric insulating layers composed of Na are stacked in the C axis direction one monolayer after another.
NaxCoO2 has a strong anisotropy in a thermoelectric property. In NaxCoO2, S⊥c/S∥c is approximately to 2, ρ⊥c/ρ∥c is approximately to 0.025.
Herein, a Seebeck coefficient and an electric resistivity in a perpendicular direction to the C axis, that is, in a parallel direction to the layered structure (with respect to each layer) are expressed by S⊥c and ρ⊥c, respectively. A Seebeck coefficient and an electric resistivity in a parallel direction to the C axis, that is, in a perpendicular direction to the layered structure (with respect to each layer) are expressed by S∥c and ρ∥c, respectively.
That is, it is believed that in comparison by ZT, NaxCoO2 has a better property in the perpendicular direction to the C axis than in the C axis direction.
Therefore, it has been considered advantageous, in view of efficiency, when the thermoelectric conversion device is configured such that a carrier or heat flows in the perpendicular direction to the C axis of the layered oxide, that is, in the parallel direction to the layered structure.
On the other hand, a thermoelectric conversion material produced according to the conventional method is a polycrystal that has no crystalline orientation. Therefore, it is essentially impossible to configure, by using such a polycrystalline material, such that the carrier or the heat flows only in the perpendicular direction to the C axis.
In addition, another factor resulting in performance deterioration is an increase of the electric resistance caused by the carrier being dispersed in crystal grain boundaries that exist in the polycrystalline material in large numbers.
Due to these reasons, it is needed to produce a thermoelectric conversion material of which crystalline orientation is aligned. As methods for producing the thermoelectric conversion material of which crystalline orientation is aligned, in the case of a thin film, for example, there is a method in which the crystalline orientation is controlled by using a single crystalline C-surface substrate of Al2O3 as a template.
To date, even when Bi2Te3 and the layered oxide are used, the performance in the conventional device configuration is not satisfactory. It requires a further improvement on the performance of the device for a full-scale practical application in the commercial use.
On the other hand, in addition to the attempts to enhance the ZT of the material itself, there has been an attempt to enhance the efficiency by improving the configuration of a device.
Shakouri et al., have proposed a thermoelectric cooling device in which a cooling-side electrode smaller in area than a radiation-side electrode is arranged (Applied Physics Letters Vol. 85, pp, 2977-2979 (2004) (reference (3)) see).
However, the material used for this thermoelectric cooling device is a material having an isotropic thermoelectric property, and thus, in the above-described configuration, there are no other effects than effects of facilitating an electric current dispersion in the material, and preventing a Joule heat returning. Therefore, the efficiency is improved by several times at most, and a device having sufficient efficiency has not been realized.
As described above, the conventional thermoelectric conversion devices have not demonstrated a satisfactory performance, and have not been able to obtain the efficiency to the extent that it is generally used in a commercial application.